new page

Drag a page break from the "Breaks and Spacers" palette to the last measure of the previous page: any following measure will go to a new page.

But also be aware, you shoudln't normally need to need to do that, any more than you normally need to do the same in a word processor. MuseScore will automatically go to the next page as necessary just as a word processor will. The only time you would normally need to explictly add a break is it you want to start another page *before* it is needed, like if you want the next movement of a piece to start at the top of a oage. And in that case, you should probably also add a Section Break from the palette, to resrt measure numebring, suppress courtesy clefs & signatures, etc.